This invention relates to a method and a device for random-accessing optical discs such as a Compact Disc, a CD-ROM and a CD-I in which data is recorded at a constant linear velocity and optical discs such as a MD (Mini Disc) in which data is to be recorded at a constant linear velocity for reading or writing of data and, more particularly, to a method and a device for accessing optical discs suitable for use in a case where random accessing to optical discs is frequently made.
For recording data in a Compact Disc, the CLV (constant linear velocity) system is generally employed for improving data memory capacity. During reproduction of data, therefore, it is necessary to change the rotation speed of the Compact Disc in accordance with an access position of a read head or an optical pick-up head on the Compact Disc. Such control is necessary also in a CD-ROM which utilizes a Compact Disc as a read-only memory. For this reason, in performing a random accessing a CD-ROM, a rotation control is frequently made for changing the rotation speed of the Compact Disc in accordance with a read position of the read head.
In a case where random accessing to a CD-ROM is made at a high speed according to the above described conventional method and device, the read head reciprocates frequently and at a high speed between the inner portion and the outer portion of a Compact Disc and, therefore, the number of revolution per unit time of the Compact Disc must be changed frequently and waiting time must be provided until the Compact Disc has attained the required number of revolution per unit time. This necessitates much time for waiting until attainment of the required number of revolution per unit time thereby hampering a high speed accessing to the CD-ROM.
This problem is particularly serious in a case where a motor of a small torque is used for rotating the disc because more time is spent for waiting until attainment of a stable revolution number than for movement of a read head with resulting deterioration in the accessing efficiency.
It is, therefore, an object of the invention provide a method and a device for accessing an optical disc capable of remarkably shortening waiting time during accessing and thereby realizing a high speed accessing to the optical disc.